The invention is directed to those fields of technology wherein laser diodes are utilized for generating laser beams, for example to electronic reproduction technology, and is directed to a method and to a circuit arrangement for correcting the light power output of a laser diode.
Laser printers and laser recorders are employed in electronic reproduction technology for recording, whereby the laser beam is being increasingly generated by laser diodes.
The light power output of such a laser diode is controlled by a driver current that is modulated by an image signal that contains the information to be recorded. The light output by the laser diode is shaped with optical means to form a light beam that is guided across the recording medium point-by-point and line-by-line with a deflection system (polygonal mirror).
For recording line information, the laser diode works in switched mode in that the light output by the laser diode is turned on and off by the driver current modulated by the image signal. In order to achieve a high recording quality, the laser diode must switch quickly and the light level must be as constant as possible in the on-time intervals. The switching behavior of the laser diode can be improved by impressing an additional Townsend current for setting a favorable operating point. Based on its very nature, the laser diode does not satisfy the demand for a constant light level in the on-time intervals, since the light power that is output is temperature-dependent, namely such that it decreases with increasing operating temperature.
During continuous operation, the gradual temperature rise in the substrate of the laser diode and the drop in light power caused as a result thereof can be compensated by controlling the housing temperature. In switched modes, however, a laser diode still has a dynamic temperature effect whose cause is the temperature change of the laser transition in the chip dependent on the modulation of the laser diode or, respectively, on the image signal. As a result thereof, the light power that is output is additionally dependent on the modulation or, respectively, on the image signal.
Even when the housing temperature is kept constant by the control, there continues to be a temperature difference between substrate and laser transition that leads to a temperature compensation process within the laser diode. This internal temperature compensation process deteriorates the switching behavior of the laser diode in such a way that the light power that is output at the turn-on time rises above the nominal level and only gradually reaches the nominal level within the on-time interval. As a consequence of this chronological curve of the light power, a disturbing lag effect which considerably influences the recording quality arises when recording an image information on the recording medium.
Various measures for correcting the light power output of a laser diode are already known.
For example, it has already been proposed to respectively measure the light power output by the laser diode within the time interval required for recording a line and to regulate the light power via the driver current dependent on the measured result.
GB-B-21 01 841 likewise also discloses that the light power that is output be measured with a photodiode (monitor diodes) integrated in the laser diode within or outside the time interval required for recording a line, to calculate correction values from the respectively measured light power, to intermediately store the correction values line-by-line in sample-and-hold circuits, and to regulate the light power via the driver current dependent on the stored correction values.
EP-A-0 141 191 further discloses that the Townsend current also be regulated in addition to the driver current in order to achieve a constant light power.
The known measures for correcting the light power output on the basis of regulation have the disadvantage that stability problems arise due to the control loops, and the switching speed of the laser diodes is reduced. Given the known measures, moreover, only one measured value is acquired in each recording line and is utilized for regulating the light power. Since the temperature changes of the laser transition and the changes in light power caused as a result thereof are dependent on the alternating frequency of the image signal and an extremely great number of changes of the image signal occur during a recording line, the changes in light power within the recording lines cannot be corrected with the precision required for a good recording quality.
US-A-4,149,071 discloses a circuit arrangement charged by a modulation signal for driving a laser diode having a compensation circuit with which disturbing changes of the light power output by the laser diode due to the dependency of the light power on the internal temperature of the laser diode are compensated, so that the output light power is proportional to the modulation signal.
The compensation circuit is essentially composed of a linear low-pass filter that approximately electrically simulates the chronological temperature curve of the laser diode dependent on the modulation signal. The output signal of the low-pass filter is then employed for correcting the driver current for the laser diode, this driver current defining the light power that is output.
Since the characteristic of a laser diode changes with the operating point, such a linear correction of the light power output by a laser diode is often insufficient when great precision is required.